SiC has a larger energy band gap compared with Si, so various techniques for production of high quality SiC single crystal suitable for semiconductor materials have been proposed. As the method of production of SiC single crystal, up to now, various diverse methods have been tried out, but the sublimation method and the solution method are currently the most general. The sublimation method is fast in growth rate, but has the disadvantage that micropipes and other defects and transformation of the crystal polytype easily occur. Opposed to this, while the growth rate is relatively slow, these defects are not present in the solution method. This method is therefore considered promising.
The method of production of SiC single crystal by the solution method maintains a temperature gradient inside the Si melt in the graphite crucible where the temperature falls from the inside toward the melt surface. At the bottom high temperature part, the C which dissolves from the graphite crucible into the Si melt mainly rides the convection of the melt to rise and reach the low temperature part near the melt surface and become supersaturated there. If causing a SiC seed crystal which is held at the tip of a graphite rod to touch the melt surface, The supersaturated C crystallizes on the SiC seed crystal by epitaxial growth as a SiC single crystal. In the present application, the “growth temperature”, “touch temperature”, etc. mean the temperature at the melt surface.
A SiC single crystal, in particular for securing good device characteristics as a semiconductor material, has to have as low a density of dislocations and other lattice defects as possible. For this reason, it is important to make the single crystal grow so as to prevent the defect density of the seed crystal from being made to increase. If causing the seed crystal to touch the melt surface, the large temperature difference between the two will cause a large stress to be applied to the touch surface region of the seed crystal and the thin single crystal starting to grow, so lattice defects will occur. These will grow and lead to defects in the final single crystal.
Therefore, to prevent the occurrence of such defects, up to now, various proposals have been made regarding the method of making the seed crystal touch the melt.
Japanese Patent Publication (A) No. 7-172998 proposes to cause the seed crystal to descend to make it touch the melt surface at the point of time when the Si melt reaches a temperature lower than the growth temperature of 1700° C. by 100° C. and then make the temperature of the Si melt rise to the growth temperature to thereby cause the seed crystal surface to slightly melt and remove the work marks and oxide film present on the surface. However, with the method of this proposal, the temperature is only raised to the single crystal growth temperature, so the C supersaturation degree is insufficient and a good growth rate cannot be secured.
Further, the following proposals have been made.
Japanese Patent Publication (A) No. 2000-264790 proposes production of a SiC single crystal by the solution method comprising causing the seed crystal to touch the melt surface (seed touch) at the point of time of a growth temperature of ±100 to 150° C., allowing the melt to stand for a while until its temperature becomes the growth temperature, and causing the touch surface region of the seed crystal and/or the thin single crystal which has started to grow on the seed crystal to melt in the melt (meltback). However, if the concentration of C in the melt reaches a saturation concentration at the point of time of the seed touch, the SiC single crystal will start to grow immediately right after the seed touch and will become a heterogeneous polytype crystal, but crystal defects will occur. In the end, it is not possible to reliably prevent the occurrence of defects due to seed touch.
Japanese Patent Publication (A) No. 2007-261844 proposes to make a SiC single crystal grow by the solution method from a melt which contains Si, C, and Cr during which time causing the seed crystal to touch the melt after holding the melt for a predetermined time after the melt temperature reaches the growth temperature.
Japanese Patent Publication (A) No. 2006-143555 also makes a similar proposal.
In each case, it is not possible to reliably reduce defects caused by making the seed crystal touch the melt surface in the seed touch.
Further, Japanese Patent Publication (A) No. 2008-159740 proposes production of a SiC single crystal by the CVD method which comprises making a heating plate rise in temperature once up to a temperature region higher than the growth temperature before the start of SiC growth to clean the surface before growth, then causing the temperature to descend to the growth temperature to grow the SiC. In the CVD method, unlike the solution method, contamination of the heating plate surface is merely removed. This contributes nothing to the reduction of defects caused by the seed touch in the growth of the SiC single crystal by the solution method.
Further, Japanese Patent No. 3079256 proposes to use the sublimation method to grow a SiC single crystal during which time firing an energy beam (CO2 gas laser beam) at the substrate or substrate holder so as to control the temperature inside the crystal during growth. This is also art for controlling the temperature profile in the crystal in the sublimation method—which is different from the solution method. It does not contribute anything to the reduction of defects due to the seed touch in the growth of SiC single crystal by the solution method.